One was mostly undamaged, and I gather that was fixed up, repainted and resold through Mack Australia. The new owner probably never knew where it came from, and thus it would be virtually impossible to trace it. The same happened with most of the bikes from Toecutters gang in the first film. They were borrowed from Kawasaki, and after the film they were returned and simply sold as new bikes.

The second Mack, the one that did the rollover, I believe was wrecked for parts. I've seen photos of it after it was pulled back onto the road, and it was pretty stuffed. Remember that it slammed into the dirt with that cow catcher attached, which would have put a lot of extra stresses on the cabin / body. I would have been very surprised if anyone had attempted to repair it.

As for the question of 2 Macks, I believe it's for the same reason they had 2 Interceptors - one for detail / close ups, one for stunts. Apparently the models are slightly different, and a few people have mentioned to me that you can spot that there are 2 different Macks being used throughout the film.